Arizona Agricultural Experiment Station 299 



the top of the loam layer and non-adherent to it, but its upper surface 

 is quite regular and smooth. Pebbles embedded in the cap are frequent, 

 and the laminae formed after the pebbles became embedded take the 

 contour of the pebbles. When a broken cap is examined in a proper 

 light, it is seen to be laminated and the laminae show various shades 

 of color, from nearly white to chocolate according to the foreign 

 material that became trapped in the lime deposit as it was forming. 

 Thus the caps could have been formed only at the surface and not by 

 any sort of subsurface evaporation. Caliche-like incrustations of con- 

 siderable thickness occur over basalt rocks on the slopes of buttes adja- 

 cent to Tucson. Fossils are absent in cahche with the exception of 

 diatoms, which the writer has found only in one specimen. From its 

 mode of occurrence caliche appears to have been formed by lime- 

 secreting organisms either in water or on surfaces that were frequently 

 wetted with limy waters. During intervals of comparative quiet when 

 little debris was being introduced and there was little interruption to 

 the growth of these organisms the smooth caps may have been laid 

 down. 



A few observations that may be corroborated by anyone during the 

 ummer months render this theory more plausible. Leaky hydrants 

 supplied with the hard Tucson city water rapidly become covered 

 with lime-secreting algae and deposits of appreciable thickness are 

 formed. These deposits cannot be due to evaporation. On the con- 

 trary flowing hydrants on the University grounds, supplied with a less 

 hard, slightly black alkaline water, form only light lime deposits. On 

 hot summer days with intense insolation caliche roads under lawn 

 sprinklers become green with algae (probably lime-secreting species) in 

 two or three hours. If these processes were continuous and extraneous 

 matter introduced, undoubtedly caliche-like deposits would be formed. 



The chemical evidence of the organic formation of caliche has 

 been less completely worked out. It appears, howeyer, that the per- 

 centage of readily soluble potassium and the potassium-sodium ratio 

 is quite high in caliche. A few comparative determinations of phos- 

 phoric acid on caliche and on the interlying and overlying loams show 

 the phosphorous content of caliche much higher than that of the 

 associated soils. If caliche were merely lime-cemented soil we would 

 expect the opposite relation to exist, phosphates being moved but slowly 

 by ground waters. Further analytical work will probably corroborate 

 these observations, and show the similarity of caliche to travertines of 

 known organic origin. 



